1. Field of the Invention
The present invention is generally directed to integrated-circuit (IC) designs. More particularly, the present invention is directed to area-reducing IC designs for use in devices (e.g., mobile telephones).
2. Background Art
Mobile telephones typically transmit and receive radio-frequency (RF) signals, but internally process lower-frequencies signals. To internally process low-frequency signals while transmitting and receiving RF signals, a mobile telephone includes circuitry to convert signals from low to high frequencies and vice versa. For example, in a transmit path of a typical mobile telephone, a voice signal is encoded at a relatively low frequency and then an RF-conversion circuit converts the encoded voice signal into an RF signal for transmission. In a receive path, a typical mobile telephone includes another RF-conversion circuit to convert a received RF signal into a lower frequency before being decoded and provided in an audible form to a user. The RF-conversion circuits in the transmit and receive paths may be included in a single IC (chip) or may be divided into separate ICs (chips).
An RF-conversion circuit in a receive path of a mobile telephone typically includes a plurality of different filters that are used during conversion of an RF signal to a lower-frequency signal. For example, FIG. 1 illustrates a plurality of example filters, including a first filter 110A through an N-th filter 110N. Each filter 110 includes a signal-path filter 102 and a feedback loop 104. Signal-path filter 102 filters an input signal to provide an output signal. Feedback loop 104 provides some type of secondary filtering, such as direct-current (DC) cancellation.
In conventional RF-conversion circuits, the filters may include large-sized resistors with good accuracy. For example, conventional mobile-telephone receivers (such as, wideband code division multiple access (WCDMA) receivers, GSM receivers, or the like) include a two-stage baseband filter. FIG. 2 illustrates an example first stage of such a two-stage baseband filter. As shown in FIG. 2, the signal path 102 is configured to filter an input signal and includes resistors R1 and Rf which are relatively small (e.g., approximately 2.4 kΩ to approximately 24 kΩ). The feedback loop 104 is configured as a high-pass filter—which causes feedback loop 104 to function as a DC-cancellation loop because any DC component at the output is collected into feedback loop 104 and is subtracted from the input. The cut-off frequency of this high-pass filter is inversely proportional to resistors R1hp included in DC-cancellation loop 104. Typically, the cut-off frequency is relatively low, meaning that resistors R1hp are relatively large (e.g., approximately 2 MΩ). Due to their large size, resistors R1hp of DC-cancellation loop 104 occupy a large percentage of the chip area (e.g., approximately 20% in conventional designs). Resistors that consume a large percentage of chip area are undesirable.
One potential solution for reducing the area occupied by the resistors of DC-cancellation loop 104 is to use n-well sheet resistors because these resistors have a smaller area footprint compared to other types of resistor. Unfortunately, n-well sheet resistors have relatively low accuracy compared to other types of resistors. Due to their relatively low accuracy, n-well sheet resistors typically should not be used in filters that require high accuracy, such as in signal-path filter 102. Although a feedback loop (like DC-cancellation loop 104) typically does not require high-accuracy resistors, the resistors of DC-cancellation loop 104 should be of the same type as signal-path filter 102 to provide a well-defined high-pass cut-off frequency. Accordingly, because n-well sheet resistors should not be used in signal-path filter 102, n-well sheet resistors should also not be used in DC-cancellation loop 104.
Given the foregoing, what is needed is a filter having a smaller area footprint compared to conventional filters, and applications thereof.